Percutaneous transluminal coronary angioplasty (PTCA) is a therapeutic medical procedure used to increase blood flow through the coronary artery and can often be used as an alternative to coronary by-pass surgery. An elongated catheter having a deflated balloon at its distal end is guided through a patient's cardiovascular system to the coronary artery of the heart. The balloon is inflated to compress deposits that have accumulated along the inner walls of the coronary artery to widen the artery lumen and increase blood flow. Typically, the balloon catheter is guided to the specific area within the vessel by an elongated guidewire. The guidewire is inserted into the patient and routed through the cardiovascular system and can be viewed on an x-ray imaging screen.
The path the guidewire follows during this procedure is often tortuous. The distal tip of the guidewire is flexible to avoid damaging the inner walls of the blood vessels which the guidewire tip contacts along the tortuous path. The distal tip is often pre-bent to a desired configuration so that the guidewire can be inserted into branching blood vessels along its path. The physician must be able to orient the tip so it can be pushed into these branching blood vessels. Examples of prior art guidewires are shown in U.S. Pat. No. 4,846,186 issued to Box et al. on Jul. 11, 1989 and U.S. Pat. No. 5,267,574 issued to Viera et al. on Dec. 7, 1993, both of which are hereby incorporated herein by reference.
Such guidewires typically have a core made from stainless steel or the like and coated with a lubricity enhancing agent, such as Teflon.RTM.. The distal end of the guidewire is often not coated as such and usually comprises one or two tapered portions which reduce the diameter of the core wire at its distal end. The distal most portion of the core wire is then flattened to form a ribbon tip which makes it easier for a physician to form it into a desired shape. A flexible coiled wire spring surrounds the distal tip of the core wire and is attached thereto. The coil separates from the core wire for a predetermined length and is attached proximal to the flattened distal portion of the core wire.
Among the difficulties sometimes encountered with conventional guidewires, is the possibility that the distal end of the guidewire may kink as it is advanced through the patient's vasculature. Kinking is the result of a plastic deformation of the guidewire and usually is characterized by a sharp deformation or point bend of the very distal section of the wire. Such a deformation may result from attempting to pass a guidewire through a relatively hard, calcified lesion, a mostly occluded vessel section or a very tortuous vascular section. The wire may kink or bend back upon itself in a condition referred to as prolapse. Thereafter, the wire may return to its original shape, or it may remain permanently deformed if, during the bending, the wire material is bent beyond its elastic limit.
Once permanent kinking occurs, the guidewire is usually discarded because it cannot be adequately straightened for use. Consequently, the procedure may have to be aborted and a new guidewire selected, reinserted, and again manipulated and advanced to the target site. Reinsertion of another guidewire increases the risk of trauma to the blood vessels. Unless great care is taken, the blood vessels can be seriously damaged.
Recently, guidewires having core wires which are made from superelastic alloys such as Nitinol.RTM. have become more popular. An example of such a wire is given in U.S. Pat. No. 5,411,476 issued to Abrams et al. on May 2, 1995, which is hereby incorporated herein by reference. The superelastic nature of the core wire allows the metal to be deformed and restrained in the deformed condition, causing the alloy to transform from an austenite phase to a martensite phase. However, once the restraint on the superelastic member is removed, the stress is reduced and the core returns to its original undeformed shape by the transformation back to the original phase.
However, as mentioned above it is desirable to provide a guidewire having a curvature or some other shape at its distal end to assist the physician in introducing, advancing and steering the guidewire and catheter to the target site in the blood vessel. One problem with guidewires made of superelastic materials is that, unlike the conventional prior guidewires, they cannot be readily formed, immediately prior to the procedure, into a shape desired for a specific procedure. This is because the superelastic property which is so desirable in prevention of kinking serves to preclude formability by the physician. Accordingly, there is a need for a guidewire that combines the advantages of guidewires formed of superelastic alloys with the ability to form or shape the distal end of the guidewire immediately prior to use by the physician.
Prior art solutions to this problem have included using a malleable safety wire extending between the tip of the core to the extreme distal end of the guidewire. Others have suggested coating the distal end of the guidewire with a malleable material. Examples of such guidewires are presented in U.S. Pat. No. 5,368,049 issued to Raman et al. on Nov. 29, 1994, which is hereby incorporated herein by reference. However, many superelastic materials, such as Ni--Ti alloys, do not weld very well, especially to malleable materials such as stainless steel. This could cause the safety wire mentioned above to detach from the core wire, possibly posing a danger to the patient. In addition, the poor welding properties of such materials make the coating suggestion impractical for manufacturing.
There has, therefore, been a need to have a superelastic guidewire with a shapeable tip which can be better manufactured and is of a simpler design. The present inventions provides such a guidewire and overcomes the shortcomings of the prior art.